Updated Koa Cors mudule to allow API being queried using the OPTIONS

This PR relates to issue: https://github.com/jembi/openhim-core-js/issues/221. 

The issie was due to the version of Koa Cors we were using. The problem we experienced was a noted issue and the module owners has corrected this in a updated version. The update to place here:
https://github.com/evert0n/koa-cors/blob/master/index.js#L91-L95